This program project continues to investigate defects in cellular and humoral immunity. Work in progress includes the study of B cell functions in normal and immunodeficient individuals. A study of the T cell message for B cells has devolved upon a biochemical characterization of this lymphokine. Studies of the genetic defects of the complement system are continuing with a detailed examination of the structural bases of the opsonic activity of the third component of complement, the factor B of the properdin system and of the peptide generated in the plasma of patients with hereditary angioneurotic edema. BIBLIOGRAPHIC REFERENCES: Parkman, R., Rosen, F. S., Rappeport, J., Camitta, B., Levey, R. L. and Nathan, D. G. Detection of genetically determined histocompatibility antigen differences between HL-A identical and MLC nonreactive siblings. Transplantation 21:110-116, 1976. Hirschhorn, R., Beratis, N. and Rosen, F. S. Characterization of residual enzyme activity in fibroblasts from patients with adenosine deaminase deficiency and combined immunodeficiency: Evidence for a mutant enzyme. Proc. Nat. Acad. Sci. USA 73:213-217, 1976.